One problem often encountered in the manufacture of crystal controlled oscillators in the HF to UHF frequency band is that some of the oscillators will not start under all of the specified operating conditions. This has been the case with oscillators of all three of the common configurations, Colpitts, Clapp and Pierce. To insure reliable starting, manufacturers of the oscillators have resorted to testing the oscillators at temperature extremes to reject non starters, or to specially selecting the crystal resonators, the transistors, or both. Obviously it is desirable, if possible, to eliminate the expense of these procedures.
The apparent reason for poor starting is that negative resistance generated by the particular transistor amplifier at small signals is not great enough to overcome the oscillating resistance of the crystal resonator. The sum of the two resistances must be no greater than zero for the oscillator to start.
It is well known that the resonator oscillating resistance is a function of the electrode structure used to launch the acoustic waves. Unfortunately, however, structural changes that tend to lower the oscillating resistance, at the same time increase the inter-electrode capacitance. This capacitance shunts the series resonant circuit of the resonator and limits the range of frequency through which the resonator can be "pulled." In fact, the ratio of the series resonant capacitance to the shunt capacitance is sometimes referred to as a figure of merit for tunability of the resonator. This ratio is particularly low in the case of surface acoustic wave (SAW) resonators.
As a result of the unfortunate tradeoff between oscillating resistance and shunt capacitance, in addition to starting problems, crystal controlled oscillators in these frequency bands normally have very narrow pull ranges. Where variable frequency is needed, such as in a phase-locked loop, known circuits are unable to take advantage of the high Q that comes with operating a crystal resonator at resonance. They use the resonator either as a reactive component or as a delay line. This usually results in low Q and substantial oscillator noise.
An object of my invention is a crystal controlled oscillator with reliable starting that can be manufactured without special product or component selection.
Another object of my invention is a variable frequency crystal controlled oscillator with a wide pull range and very low noise because the crystal resonator operates at resonance.